We propose to study, using a variety of electrophysiological techniques, the differentiation of excitable membrane properties during neuronal dvelopment. Two types of model syytems are currently being studied. The first consists of single sympathetic neurons isolated from the superior cervical ganglion of rats or chick embryos and grown in culture; the second, a clonal cell line PC12 originally isolated from a rat pheochromocytoma and grown in culture. Membrane properties which are being investigated include among others the expression and behavior of excitable conductances for Na ion, Ca ions, K ion as well as a variety of other K ion conductance mechanisms.